1. Field
The field of the disclosed subject matter is detection of coking in refinery equipment. More specifically, the field of the disclosed subject matter is the use of optical sensing networks to detect coking in refinery equipment.
2. Description of Related Art
The processing of a hydrocarbon-containing feed streams at elevated temperatures in a processing zone, such as a furnace, heat exchanger, distillation tower or other refinery equipment, can result in the formation of carbonaceous substances which can deposit on surfaces of the equipment. Such carbonaceous substances are generally referred to as “coke” in the fields of petroleum refining and petro-chemical processes.
Coke deposition on equipment surfaces can alter the operation of the equipment, usually in an undesirable manner. For example, feed streams are heated in a furnace before being introduced to distillation columns. Formation of coke can result in a blockage of tubes in the furnace, as well as the blockage in the transfer lines from the furnace to the distillation column. Additionally, coking often occurs in the column itself, typically within wash beds or at interfaces between different types of packing or the like. Coke can also occur in the bottom of the tower and plug liquid outlets and pump strainers, causing pump cavitation and damage.
After the process of distillation and vacuum distillation, petroleum refining operations in which crude oil is processed frequently produce residual oils. Many oil refineries recover valuable products from heavy residual hydrocarbons. Residual oil, when processed in a delayed coker, is heated in a furnace to a temperature sufficient to cause destructive distillation. In this manner, a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields petroleum coke, a material composed mostly of carbon.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. Coking typically begins in a furnace, continues in a transfer line, and finishes in the coke drum. The delayed coking process employs a furnace that operates at temperatures as high as about 1000° F., roughly 50 to 100° F. higher than the operating temperature of the coke drum. The high furnace temperatures can promote the rapid formation of insoluble coke deposits on the furnace tubes and transfer lines.
When coke deposits reach excessive levels, the operation must be shut down and the furnace de-coked. Frequent interruptions for cleaning can lead to high operating costs due to increased amounts of time the operation is off-line, in addition to the cost of the de-coking operations.
The chemical and physical factors involved in the formation of coke have not been fully elucidated. That is, the propensity of certain feed streams to form coke and the rate at which coke is actually formed and deposited in industrial plant such as petroleum refineries and petrochemical works is not currently predictable. As a result, maintenance schedules which take account of deterioration in process and equipment performance due to coke are not necessarily accurate or efficient. Hence, there is a need to detect and to predict coking in refinery equipment or the like.
Detection of coking in refinery equipment, including distillation towers, distillation tower bottom circuits, distillation tower feed furnaces, and coker feed furnaces and transfer lines, has been addressed with a variety of techniques. For example, measurement of pressure drop has been used to detect coking. This technique has been problematic, such as for vacuum tower wash beds, where the pressure drop is typically only on the order of a few mmHg in these wash beds when coking occurs. Thus, pressure measurement is highly unreliable. Similarly, temperature differentials between bulk temperatures have also been used to detect coking. However, this technique involves a gross measurement and thus not necessarily accurate.
The use of various external mechanical probes, which measure light emitted by the refinery equipment, for the detection of coking and fouling have been disclosed. For example, in U.S. Pat. No. 4,402,790 (the '790 patent), a mobile machine to gather coke oven flue temperatures is disclosed. The machine includes a probe head attached to an optic cable to transmit infrared radiation to photoelectric detection-conversion cells. The machine also contains a reeling machine for movement of the cable. The probe is manually directed to enter a coke oven flue and the probe head contains viewing ports for the terminal ends of the fiber optic cable to sense infrared radiation. The infrared radiation is transmitted to photoelectric detection-conversion cells for processing to calculate temperature. The machine disclosed is portable and the probe head must be inserted into a coke oven flue to obtain a temperature measurement. Moreover, the machine incorporates an extrinsic fiber optic sensor—that is, infrared radiation from inside the component of refinery equipment is transmitted through the optic cable to photoelectric detection-conversion cells.
By contrast, U.S. patent application Ser. No. 12/024,251, published as US Patent Application Publication No. 2008/0185316, is directed to a mechanical probe with an extrinsic fiber optic sensor to detect light scattering for the detection of flocculation of quench oil. Particularly, U.S. patent application Ser. No. 12/024,251 discloses the use of transmission, reflectance, and attenuated total reflectance probes to detect an increase in light scattering resulting from addition of precipitant to a quench oil sample.
Hence, there remains a need to provide easier and more effective solutions for detecting the advent and progress of coking in refinery equipment in view of the continued desire to provide enhanced mitigation strategies. Furthermore, there remains a need for a detecting system that allows for proactive operations to mitigate coking